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How big a reduction in braking power is it?

Can you notice the difference? Does it do this while you are already braking and the batteries end up full? (in other words, does the braking action change while you are braking?)

I'm in Atlanta, so the winter doesn't get abnormally cold. But the current gauge goes down to 60 watts and I've seen the limited hover as near as 15 from the cold. The only worse I've seen is when the battery was full from a "Max Range" charge and braking was nearly disabled completely. When it's like that, I simply switch back to the normal brakes.

One thing to point out here is that driving in the Model S is "one foot driving." That is, the regenerative braking force enables as you reduce pressure on the accelerator. If done right, you rarely need to use the mechanical brakes at all. So, there's no actual reduction in braking capacity. The Model S doesn't just careen down the road. This simply affects the regenerative braking system.

Any reason they can't just route the current generated when battery is full to a big resistor/heatsink?

That would seem to avoid the problem of changing braking dynamics, and potentially save wear on friction brake components (pads, discs, etc)...

It's kind of a lot of power (hundreds of kilowatts for a panic stop from 60mph), so a reliable resistor bank would probably add more weight/bulk than it buys in reduced brake wear.

it's not really about the reduced brake wear so much as avoiding the sudden change in stopping distance when the battery stops charging.

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